The distinction between viable but ischemic myocardium and irreversibly scarred myocardium by exercise thallium-201 (Tl) scintigraphy is often imprecise, as many Tl perfusion defects that develop in ischemic myocardium during exercise do not normalize on subsequent 3-4 hr. redistribution (RD) images. We have recently demonstrated that 1 mCi of Tl reinjection (RI) at rest immediately after stress-RD imaging identifies ischemic but viable myocardium in regions that would otherwise have been interpreted as scar on the basis of apparently irreversible defects on RD imaging. That the uptake of Tl after RI rep[resents viable myocardium has been verified by improved regional perfusion and wall motion after coronary artery revascularization, preserved regional metabolic activity on positron emission tomography (PET), and by preserved regional systolic wall thickening by gated magnetic resonance studies. Furthermore, 24 hour delayed imaging following RI provides no additional insights regarding myocardial viability in 94% of patients with irreversible Tl defects at RD. To determine if Tl RI is also more sensitive than rest-RD imaging for detecting viable myocardium, we studied 20 pts with coronary artery disease by exercise-RD Tl imaging followed by 1 mCi Tl RI and compared the images to rest-RD Tl images in the same patients 1-2 weeks later. Concordance and discordance between rest-RD imaging and Tl RI imaging were compared to metabolic marker by PET. The results indicate that rest-RD and Tl RI imaging provide concordant information regarding myocardial viability in the majority of regions, but rest-RD underestimates viable myocardium in regions with severely reduced Tl activity when compared to Tl RI and PET.